Immersion objectives for microscopes can include an immersion fluid introduced between the objective and the preparation, thereby increasing the achievable resolution, so that there is no empty magnification, not even at high magnifications. A further effect is the suppression of contrast-reducing reflections, in particular when an immersion oil is used which has approximately the same refractive index as glass. An oil immersion objective having a numerical aperture of NA=1.4 produces, for example, a maximum resolution of 217 nm at a wavelength of 500 nm. The upper threshold for the numerical aperture is dictated by the refractive index of the immersion medium, which is typically 1.518 with oil.
A field in which oil immersion objectives are commonly used is fluorescence microscopy. Care should be taken in this case to ensure that the immersion oil used does not have its own fluorescence.
Conventional oil immersion objectives are generally distinctive in that the lens group closest to the object (“front lens group”) consists of at least two cemented lenses. Upon cementing, adjacent lens faces, which must have the same radii, are stuck together using a thin, transparent cement layer. The cement layer, which is mainly made of synthetic resin, is also used to prevent total internal reflection (and reflections in general) at glass-air surfaces between two lenses without cement. The cements used are, however, disadvantageous in a number of microscopy applications. In some microscopic methods, in particular in fluorescence microscopy, high-performance laser illumination is operated. Particularly in methods which focus the laser light into the objective, this leads to a higher intensity within the objective, especially in the region of the front lens group. Cements used at this location may incur damage when irradiated at such high intensities. This results in turbidity at the damaged cement site.
Furthermore, cements have a specific own fluorescence. This is disadvantageous in methods in (wide-field) fluorescence microscopy, since much of the fluorescence light occurring in the cement also enters the image and therein reduces the contrast of the useful light from the fluorescing sample.
U.S. Pat. No. 7,199,938 B2 discloses an apochromatic immersion objective for microscopes which has an object-side front lens group composed of cemented lenses. This front lens group consists of a planoconvex lens element, the planar surface of which faces the object, and of a meniscus lens element, the concave face of which faces the object-side and is connected to the convex side of the planoconvex lens element by means of cementing. The objective envisaged in said document is intended to reach a numerical aperture of NA>1.4.
US 2002/0154414 A1 discloses a microscope objective of high aperture for immersion applications, in particular TIRF (Total Internal Reflection Fluorescence). The front lens group comprises a first lens of positive refractive power having a planoconvex surface, the planar face facing the object. This lens is cemented to a second lens, which surrounds the convex face of the first lens and has negative refractive power.
Furthermore, U.S. Pat. No. 7,046,451 B2 discloses an oil immersion objective for microscopes having a numerical aperture of NA>1.45, the first, object-side (front) lens group of which likewise comprises a first planoconvex lens, the planar face of which faces the object, the planoconvex lens being cemented to a meniscus lens, the convex face of which faces the image-side. TIRF microscopy is cited as the field of application in this case too. The numerical aperture is intended to be NA≧1.46 in this case.
A known construction principle, which is also shared by the above-mentioned oil immersion objectives, for producing high apertures is to embed the planoconvex, object-facing lens in a second lens by means of cementing. Yet cementing lenses is marred by the above-mentioned disadvantages.